Photonics devices are widely prevalent in today's world. One issue in optical transport for optics communication, processing, or data transfer is efficiency when coupling radiation from one medium to another, e.g. from an optical fiber to a photonics device or vice versa. A number of coupling arrangements are available at present. One often used coupling element is a grating element. In order to optimise coupling efficiency, coupling materials and coupling configurations have been suggested.
Some systems make use of the silicon photonics platform. In the silicon photonics platform, use can be made of SiN as gratings in a SiN waveguide, having the benefit that SiN has low losses, low thermal dependency and high tolerance to fabrication errors. Furthermore, the silicon photonics platform offers the advantage that active devices can be easily incorporated. In order to increase the efficiency of the grating coupler, one suggestion is to use a mirror layer under a silicon photonics grating coupler below the buried oxide (BOX) layer of a silicon-on-insulator (SOI) wafer. It was demonstrated that this increases the coupling efficiency of the grating from a single mode fiber to a single mode silicon waveguide. Such a mirror can, for example, be a distributed Bragg reflector made of a series of alternating layers of materials having a large refractive index difference (for instance SiO2 and Si or SiO2 and SiN) or a reflective metal layer such as aluminum. The presence of the reflector increases the amount of light reflected at the BOX/Silicon substrate interface towards the grating, thus increasing the amount of light coupled. More particularly, only part of the light that hits the grating will couple to the waveguide while the other part will go through. When a reflector is present, the part going through the grating will be reflected back towards the grating having a second chance to couple.
Nevertheless, implementing the reflector below the buried oxide of the silicon-on-insulator, results in the fact that non-standard, custom made silicon-on-insulator wafers are to be used, rendering the manufacturing of the photonics device less trivial.
Alternative methods for increasing efficiency are known. In a first example, the coupling efficiency of the grating coupler is increased by applying a silicon grating by locally increasing the silicon waveguide thickness. Nevertheless, the latter requires tight control of the geometrical dimensions for controlling the grating spectral response. In another known example, a mirror is implemented under the grating by performing a substrate removal and metal deposition.
Consequently there is still a need for an efficient and easily manufacturable photonics coupling device.